Process and device for feeding oxidizing agent into a combustion system

ABSTRACT

A process and device for feeding oxygen-containing gas into the firebox of a combustion system is disclosed. A sub-quantity of the flue gas stream exiting from the combustion system (recycle gas) is subjected to pressure elevation and subsequently recycled to the firebox. An oxygen-containing gas which is present at a sufficiently high pressure is added to the recycle gas downstream of the pressure elevation.

This application claims the priority of German Patent Document No. 10 2006 025 681.6, filed Jun. 1, 2006, the disclosure of which is expressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a process for feeding oxygen-containing gas into the firebox of a combustion system, a sub-quantity of the flue gas stream exiting from the combustion system (recycle gas) being subjected to pressure elevation and subsequently recycled to the firebox, and also to a device for carrying out the process.

The flue gases formed in the combustion of carbonaceous energy sources such as coal or petroleum contain, in addition to water and carbon dioxide, a number of substances harming the environment, such as nitrogen oxides (NO_(x)) and sulphur dioxide (SO₂), for example. In industrial energy generation plants, therefore, inter alia flue gas cleaning installations are connected downstream of the combustion systems, in order to decrease the pollutant fraction in the flue gas. Flue gas cleaning, such as desulphurization, dedusting and denitrification, for example, is included among the secondary measures which make the entire energy conversion process more environmentally friendly. However, measures can also be taken to act in a primary manner, in order to substantially minimize the formation of the corresponding pollutants as early as in the combustion process.

To comply with the progressively lower emitting pollutant values and to lower the expense for secondary measures, primary measures for avoiding pollutant formation are becoming increasingly important. In the avoidance of dust formation, these principally consist in the combustion conditions, and in the reduction of sulphur-containing pollutants, in the selection of the fuels.

Great efforts for reducing NO_(x) emissions are also being taken. Seven different nitrogen oxides are known of which nitrogen monoxide (NO) and nitrogen dioxide (NO₂) are the most stable, and therefore essentially make up the NO_(x) load of flue gases. Nitrogen monoxide is principally formed solely from the nitrogen of the fed combustion air at temperatures above 1000° C. and at sufficiently high oxygen concentration. Since the combustion air consists of 78% nitrogen, the nitrogen oxides formed from atmospheric nitrogen make up a majority of the pollutant load. Depending on the fuel and type of firing, the NO_(x) content in the flue gas is between 500 and 2000 mg/m_(N3).

In the laid-open application DE 10356701, a process is described for combustion of fossil fuels in a steam generator. The combustion air here is replaced by an oxidizing agent which is essentially composed of carbon dioxide (CO₂) which is separated off from the flue gas formed in the combustion, and oxygen, and the combustion is carried out with the exclusion of air. Because of the substantial lack of nitrogen in the oxidizing agent, the flue gases only contain the amounts of NO_(x) which are formed from the nitrogen bound in the fuel, and consist virtually exclusively of water and CO₂.

The decisive reason for replacing the combustion air by an oxidizing agent which is essentially composed of carbon dioxide (CO₂) and oxygen is, however, the possibility of being able to separate off readily, after combustion with fossil fuels, the CO₂ by condensation of water and to sequester it. This aspect is increasingly gaining importance, since in the context of worldwide climate change, the CO₂ emissions which are formed by processing non-renewable energy sources must be reduced. The Kyoto protocol controls the permissible CO₂ emissions of the individual member countries and specifies the emission reductions to be achieved in forthcoming years.

The object of the present invention is to specify a process and also a device of the type described at the outset which make it possible to feed oxygen-containing gas with low energy and control expenditure into the firebox of a combustion plant.

This object is achieved according to the invention in terms of the process by the fact that an oxygen-containing gas which is present at a sufficiently high pressure is added to the recycle gas downstream of the pressure elevation.

The pressure of an oxygen-containing gas is considered to be “sufficiently high” in this case when it is at least sufficiently above the pressure which the recycle gas has downstream of the pressure elevation such that solely owing to the difference between the two gas pressures the maximum amount of oxygen-containing gas required for the combustion process flows into the recycle gas and is homogeneously mixed with this, and with gas streams possibly fed to the recycle gas at other points.

In order to be able to feed the correct amount of an oxygen-containing gas which is present at a sufficiently high pressure to the combustion system in any operating state, one embodiment of the process according to the invention provides that the oxygen-containing gas is conducted via a throttling element which can be set in a logical manner, a pressure drop being generated via the throttling element which is of a size such that the oxygen-containing gas flows in the correct amount into the recycle gas and is homogeneously mixed with this, and with gas streams possibly fed to the recycle gas stream at other points.

An oxygen-containing gas which is present at a pressure which is equal to or lower than the “sufficiently high pressure” is according to the invention added to the recycle gas upstream of the pressure elevation. If appropriate, the pressure of the oxygen-containing gas is increased or reduced in order to achieve the oxygen-containing gas flowing into the recycle gas in the amount required for the combustion process, and being mixed with it.

As oxygen-containing gases, according to the invention air and/or oxygen and/or nitrogen-enriched oxygen are introduced into the recycle gas, preferably homogeneously mixed with it, which is subsequently introduced into the firebox.

In a further development of the process of the invention, it is proposed that oxygen or nitrogen-enriched oxygen is fed from an air fractionation system which is expediently operated in the immediate vicinity of the combustion system.

Preferably, the process of the invention is used for operating a combustion system in which the flue gas is cleaned of ash and water before a part of the flue gas is recycled as recycle gas.

Particularly preferably, the combustion system is the steam generator of a thermal power station.

The object in question is achieved according to the invention in terms of the device by the fact that an oxygen-containing gas which is present at a sufficiently high pressure can be fed via a mixing system to the recycle gas downstream of the pressure elevation.

Preferably, the mixing system has at least connections for the oxygen-containing gas and recycle gas, and at least one outlet from which can be taken off a substantially homogeneous gas mixture consisting of the oxygen-containing gas, recycle gas and any gas streams fed to the recycle gas stream at other points.

Particularly preferably, the connections to the mixing system are positioned with respect to one another in such a manner that an at least approximately homogeneous mixing of the media respectively introduced there among one another is ensured. Via suitable positioning of the connections, the flow behavior of the incoming gases can be influenced, as a result of which the mixing can be controlled. Mixing chambers have proved particularly advantageous, for example, which have connections which effect tangential inflow of gases.

An expedient embodiment of the device according to the invention provides that, in the immediate vicinity of the combustion system, an air fractionation system is arranged from which an oxygen-containing gas which is oxygen or nitrogen-enriched oxygen can be obtained at a sufficiently high pressure. Preferably, the air fractionation system is a cryogenic air fractionator or a pressure-swing adsorption system (PSA).

In order to be able to add oxygen-containing gas to the recycle gas at any time in the amount required for the combustion process, in the flow path of the oxygen-containing gas, a suitable throttling system is provided which expediently can be set in such a manner that the oxygen-containing gas, on its flow through the throttling system, at all times experiences a suitably high pressure drop.

A preferred embodiment of the device according to the invention provides that the combustion system is the steam generator of a thermal power station.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in more detail hereinafter with reference to an example shown diagrammatically in the FIGURE, in which a recycle gas is mixed with air and oxygen to give an oxidizing agent and recirculated to the combustion process.

DETAILED DESCRIPTION OF THE DRAWING

In the firebox F of a combustion system V, by combustion of a fuel fed via line 1, a flue gas is generated, the sensible heat of which is for the most part utilized in a following process which is not shown. The cooled flue gas is taken off from the combustion system via line 2 and introduced into the flue gas cleaning system R from which, cleaned of ash and water, it is removed via line 3. A part of the ash- and water-free flue gas subsequently flows via line 4 to a flue gas treatment and CO₂ sequestration (not shown), while the remaining residue is recycled via line 5 as recycle gas upstream of the combustion system. Owing to the pressure drops in the combustion system, the flue gas cleaning system and the piping, the recycle gas, upstream of the compressor G, is at a pressure which is lower than atmospheric pressure. Via line 6, an air stream, the magnitude of which is controlled via the adjustable control element a, is introduced into the recycle gas stream 5 upstream of the compressor G. The gas stream thus generated which consists of air and recycle gas is introduced into the compressor G via line 7, where it experiences pressure elevation to a value which is markedly higher than atmospheric pressure and is passed on via line 8.

In the air fractionator L, oxygen is generated at a pressure which is higher than the pressure of the gas stream consisting of air and recycle gas in line 8. Via line 9 an oxygen stream, the magnitude of which is controlled via the adjustable control element b, is passed from the air fractionator L and introduced into the gas stream consisting of air and recycle gas downstream of the compressor G. The gas stream thus generated is fed via line 10 to the combustion system V, a substantially homogeneous gas mixture being generated which is introduced as oxidizing agent into the firebox F.

The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 

1. A process for feeding oxygen-containing gas into a firebox of a combustion system, a sub-quantity of a flue gas stream exiting from the combustion system as a recycle gas is subjected to a pressure elevation and subsequently recycled to the firebox, wherein an oxygen-containing gas which is present at a sufficiently high pressure is added to the recycle gas downstream of the pressure elevation.
 2. The process according to claim 1, wherein the oxygen-containing gas is air and/or oxygen and/or nitrogen-enriched oxygen.
 3. The process according to claim 2, wherein the oxygen or nitrogen-enriched oxygen is obtained from an air fractionation system.
 4. The process according to claim 1, wherein the recycle gas is cleaned flue gas.
 5. The process according to claim 1, wherein the oxygen-containing gas is fed into the firebox of a steam generator of a thermal power station.
 6. A device for feeding oxygen-containing gas into a firebox of a combustion system, wherein a sub-quantity of a flue gas stream exiting from the combustion system as a recycle gas is subjected to a pressure elevation and subsequently recycled to the firebox, wherein an oxygen-containing gas which is present at a sufficiently high pressure is fed via a mixing system to the recycle gas downstream of the pressure elevation.
 7. The device according to claim 6, wherein in an immediate vicinity of the combustion system, an air fractionation system is provided from which the oxygen-containing gas which is oxygen or nitrogen-enriched oxygen is obtained at the sufficiently high pressure.
 8. The device according to claim 7, wherein the air fractionation system is a cryogenic air fractionator or a pressure-swing adsorption system (PSA).
 9. The device according to claim 6, wherein the combustion system is a steam generator of a thermal power station.
 10. A method for feeding an oxygen-containing gas into a firebox of a combustion system, comprising the steps of: removing a quantity of a flue gas exiting from the combustion system; adding air to the quantity of the flue gas to form an air/flue gas stream; compressing the air/flue gas stream; adding a gas containing oxygen to the air/flue gas stream to form the oxygen-containing gas; and supplying the oxygen-containing gas to the firebox.
 11. The method according to claim 10, wherein the air/flue gas stream is compressed from a pressure which is lower than an atmospheric pressure to a pressure that is higher than the atmospheric pressure.
 12. The method according to claim 10, further comprising the step of removing ash and water from the flue gas exiting from the combustion system prior to the step of removing the quantity of the flue gas.
 13. An apparatus for feeding an oxygen-containing gas into a firebox of a combustion system, comprising: a recycle line, wherein a quantity of a flue gas exiting from the combustion system is removed; an air supply line coupled to the recycle line, wherein air is added to the quantity of the flue gas by the air supply line to form an air/flue gas stream; a compressor, wherein the compressor compresses the air/flue gas stream; an oxygen containing gas supply line, wherein the oxygen containing gas supply line adds a gas containing oxygen to the compressed air/flue gas stream to form the oxygen-containing gas; and an oxygen-containing gas supply line, wherein the oxygen-containing gas supply line supplies the oxygen-containing gas to the firebox.
 14. The apparatus according to claim 13, wherein the oxygen-containing gas supply line is coupled to an air fractionator.
 15. The apparatus according to claim 13, further comprising a flue gas cleaning system coupled to the firebox in a downstream flow direction of the flue gas.
 16. The apparatus according to claim 15, wherein the flue gas cleaning system removes ash and water from the flue gas.
 17. The apparatus according to claim 16, wherein the recycle line is disposed in the downstream flow direction from the flue gas cleaning system.
 18. The apparatus according to claim 13, wherein the compressor compresses the air/flue gas stream from a pressure which is lower than an atmospheric pressure to a pressure that is higher than the atmospheric pressure. 